Intracellular levels of cAMP and cGMP were measured by radioimmunoassay in mouse spleen cells settled onto surfaces coated by antigen-antibody complexes or F(ab')2 complexes in the presence or absence of a mitogenic concentration of LPS cAMP levels were consistently elevated (1.5x) after 30 minutes of contact with immobilized complexes; however, F(ab')2 complexes did not induce this change, nor did LPS affect the cAMP levels. No significant change in cGMP levels was found to be induced by immobilized complexes. Some preparations of gangliosides were found to be mitogenic for mouse spleen cells. Purified ganglioside fractions which were non-immunogenic inhibited LPS-induced mitogenesis but not PHA induced mitogenesis. MLC type reactions between lymphocytes and tumor cells were shown to be enhanced by inclusion of dextran and polyethyleneglycol in the culture medium. These results were interpreted as being due to enhanced lymphocyte-tumor interaction via specific receptors.